Spark ignited internal combustion engines are provided with spark timing control systems which control the timing of mixture igniting spark events in the engine combustion chambers with variations in engine speed and load. This has generally been accomplished with the aid of mechanically or electronically stored functions of engine spark timing relative to a reference crank angle position as a function of engine speed, load or other factors. Examples of such systems range from the familiar centrifugal and vacuum advance mechanical systems to more recent computerized spark timing control systems utilizing lookup tables or stored function generators.
It has been more recently suggested that optimal control of engine spark timing provides for generation of the spark initiation in each cylinder such that peak combustion pressure occurs at a constant predetermined crankshaft angle relative to the reference crankshaft rotational position. There have been a number of spark timing control systems suggested recently which are based on this control philosophy and which sense the timing of peak combustion pressure in at least one engine combustion chamber, compare it to the desired timing of this event and generate an error signal in a feedback control system to vary engine spark timing so that the error is always tending toward zero.
The accuracy of this method of spark timing control depends in part upon the accuracy with which the peak pressure can be detected. In a normal good combustion event, the pressure within the combustion chamber builds up quickly to a single peak and then falls rapidly again. This pressure rise creates a similar rise in strain on one or more of the engine head bolts, since increased cylinder pressure tends to lift the engine head from the block. Thus a strain sensor associated with a selected head bolt may potentially be used to generate a useful cylinder pressure signal for spark timing, with consequent simplification of equipment and design and cost savings. The resulting signal may be differentiated to determine the precise moment of maximum cylinder pressure. However, the signal is liable to be contaminated with noise at a higher frequency than that of the pressure curve, which noise makes the determination of the precise maximum more difficult. Therefore, it is advisable to provide a smoothing noise filter in a standard manner to the output of the pressure sensor or differentiator so that the precise moment of the maximum can be determined.
However, because the environment of this spark timing control system is an internal combustion engine, the rotational speed of which can vary over a range of approximately 10 to 1 or more, the characteristics of the pressure signal are greatly affected by variations in this engine speed. I have found that both the characteristics of the rising and falling waveform and the characteristics of the imposed background noise vary with engine speed.